1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly, to a method of fabricating semiconductor devices having aluminium gates.
2. Description of the Related Art
With the continuous development of semiconductor fabrication techniques, the size of a semiconductor device is continuously shrinking. The high-k metal gate gradually becomes the mainstream of the development of current semiconductor techniques, and an aluminium gate is favoured for its excellent characteristics.
At present, in a typical gate-last fabrication process, since a high-k insulating material will be subjected to a series of standard processes, including metallic silicides and silicon nitride stress engineering (double-stress layer), depositions are implemented prior to gate formation, then gaps between those gates are filled with oxides, and a chemical mechanical planarization (CMP) process is resorted to for removing excessive oxides on the surfaces of the gates. Next, gate materials for NMOS and PMOS are removed with a single process, and metal gate materials are deposited thereafter. In practice, it is required for metal gate depositions to fully cover the steps with zero damage of high-k materials. Finally, a chemical mechanical planarization technique is adopted to remove excessive metals. In order to guarantee product quality, the whole procedure is continuously monitored and cleaning processes are implemented as appropriate. When the above operations are accomplished, an insulating medium is deposited on the wafer and contact holes are formed via etching.
In the above processes, after removing excessive metals through the CMP technique, deionized water is commonly used to clean the wafer surface which has just been grinded, so as to remove retained abrasive materials. Since aluminium has a relative higher activity, when deionized water comes into contact with the surface of Al gates, it can capture electrons from Al gates and turn into H2. Thus, corrosion occurs on the surfaces of Al gates (mainly in terms of pits). If the Al gates contain impurities therein, such corrosion can be fairly serious.
FIGS. 1A-1D show scanning electric microscopy photos of the surfaces of Al gates after cleaning with deionized water. It can be seen from FIGS. 1A-1D, that pits 101 primarily occur on grain boundaries in aluminium (because impurities and defects mainly exist on those grain boundaries). It also can be seen from the FIGS. 1A-1D, that the number of the pits is very large, approximately up to hundreds or thousands. During the device fabrication, if these pits occur at gate areas, the performance of the semiconductor device can be greatly degraded and the reliability of the semiconductor device may be negatively influenced.
Therefore, a new technique is required to resolve any problem identified in prior art.